In typical commercial electrographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
One type of fuser assembly, utilized in typical reproduction apparatus, includes at least one heated roller and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member.
During the operation of the reproduction apparatus, the fuser assembly operates in various modes having substantially different temperatures. That is for example, between job runs the heated fuser roller will be in a substantially equilibrium condition where there is at most only a small temperature gradient between the outer surface of the fuser roller and the inner core. Then when the job run begins energy (heat) is removed from the fuser roller to the reproductions being fused. As a result, the temperature at the outer surface of the fuser roller droops very quickly. Since the temperature droops substantially from the desired optimum operating setpoint temperature, the logic and control for the reproduction apparatus must turn on the fuser heating device to bring the fuser roller up to its desired operating temperature. However, depending upon the thickness of the fuser roller, there is a time lag until the fuser roller surface receives enough energy to get back to the desired optimum fusing temperature. Furthermore, due to the time lag, the fuser roller may receive a quantity of energy in bringing the roller surface up to the desired operating temperature which will cause an overshoot such that the surface temperature exceeds the desired operating temperature. During the time lag, the droop, or overshoot, in surface temperature of the fusing roller may cause inferior fusing quality.
In order to maintain the fuser roller temperature as close as practical to the desired operating temperature, a control mechanism including a temperature control sensor is provided to send signals that are representative of the fuser roller's surface temperature, to the logic and control unit for the reproduction apparatus. The temperature signal is processed by the logic and control unit, and if the temperature is above or below the set optimum operating value, the logic and control unit changes the available power sent to the fuser roller heater to adjust the fuser roller temperature. A good temperature control sensor mechanism must operate repeatably from sensor to sensor, must be accurate, and must exhibit a fast response. It has been now been determined that the air flow surrounding the temperature sensor has an adverse effect on the operation of the sensor.